A Guide to the Golden Age of the Ampeg B-15: 1960-1980

In 1960, The Ampeg Company, Inc. introduced the first combination amplifier dedicated to the bass guitar: the B-15. Conceived in 1958 by the late legend Jess Oliver (aka that dude who first put reverb into a guitar amp), the B-15 grew up alongside our modern music industry. Like a lot of the popular sewing machines at the time, the B-15 stored inside of itself, as players moving from gig to gig only had to flip over the amp head to get moving. From its early days as a union-provided amp on the Manhattan Bass Club scene to its storied history inside the world’s elite studios, the Fliptop B-15 is iconic and inescapable.

Bass players of every genre and decade have favored the different flavors of the B-15 as a trusty sidekick in some of the most memorable performances in recorded history. Rick Danko used a B-15 with The Band and even used a B-15S on The Last Waltz. Motown fixture and demigod James Jamerson used a B-15 for all of his live performances as well as the majority of his non-Motown sessions in Los Angeles. As Eddie Kramer recounts, John Paul Jones would wheel his B-15 into their Led Zeppelin recording sessions. Hell, even Jimi Hendrix was known to plug into a B-15 or two, like during this performance on Dick Cavett.

Jimi Hendrix performs on The Dick Cavett Show

The evolution of the B-15 is truly fascinating. Though every B-15 I’ve come across has an undeniable mojo to it, it’s rare to find two B-15s that sound exactly the same. This article will examine those differences chronologically to help better understand what you can expect from the dusty Portaflex at your local shop. Though you can easily find out when an amp was produced by the serial number, you’ll start to see that Ampeg switched model numbers mid-year, and the distinctions aren’t always cut and dried.

B-15

The Portaflex B-15 model was first introduced in 1960 for $355. Geared toward acoustic bassists, this initial model featured extra porting options for use with upright basses. These B-15s are most easily recognized by the random flair navy tolex, the distinctive power knob, their shared tone control circuit, and the lack of a lucite light up logo.

At the outset, the B-15’s preamp section consisted of three 6SL7 preamp tubes and two 6L6GC power tubes. The initial design employed a 5U4GB tube rectifier, and even though a lot has changed cosmetically, the only change Jess Oliver made to the B-15’s actual circuit was the rectifier. The first B-15s were self-biased and outputted 25 watts of tube power.

Ampeg B-15

At this point, the two channels had independent volume controls but actually shared an equalization circuit. Using the Baxandall-style tone control circuit, Ampeg’s bass and treble controls were located between the second and third 6LS7s. The initial B-15 featured two channels with three input jacks — one for each channel — and an extra “Stereo” input jack that sent signal to both channels simultaneously.

The first B-15s generally housed a Jensen P15N speaker, brilliantly set in Jess Oliver’s patented double-baffle design. The original design placed the speaker between two baffles with 15-inch ports, separated by ⅜ of an inch. The outer baffle had a 15-inch circular cut out and a bar across the center of the speaker cone. The speaker was mounted to the inner baffle, which was fixed to the cabinet. This inner baffle featured eight oblong ports (two above, two below, two on each side) that passed the speaker’s rear wave output through the ⅜ of an inch space and through the outer baffle’s 15-inch opening, where this rear wave meets with the front wave completely in-phase.

Ampeg B-15 Patent
James Jamerson

The first edition B-15 only lasted about eight months before acquiring a few key controls, and a little sister amp, the B-12N. This updated B-15 would be dubbed the B-15N. In case you were curious, the “N” stands for “New.”

James Jamerson was a notable early Ampeg endorser, though the vast majority of Motown recordings were DIed. Check out this live picture of him with his first edition B-15. You can clearly see by the random flair tolex on the sides of the amp head and the lack of a light-up Ampeg logo that this amp was either an original B-15, or an early model B-15N (late 1960-1961).

B-15N

This new B-15 retained the same random navy flair tolex, yet added in two separate channels with separate bass and treble tone controls, an updated negative feedback resistor (which was changed from 6,800 ohms to Stereo input, and introduced a second jack on the first channel. This updated B-15 design would become the cornerstone for the rest of Ampeg’s B-15 output.

A few months later in 1961, the B-15 received three important features before the model number was updated: its hallmark light-up lucite insignia that would last until the amp was remodeled in 1968 and its tilt-back rod, which only lasted until 1964. These amplifiers from 1961-1962 are extremely rare, and you can tell them apart from any other B-15 because of the combination of navy random flair tolex and a lucite light up logo.

B-15NB

1962 brought some significant changes to the B-15. First, and perhaps least functionally, Ampeg finally started to use its distinctive blue checkered vinyl. This aesthetic would last until 1968. Two, Ampeg briefly shifted its tube rectifier to a solid-state diode rectifier. The introduction of a solid-state component brought B+ Voltage up and eliminated the need for the 5-volt filament winding. Ampeg also upgraded the multi-section can capacitor, though this change only held until 1964, when Ampeg returned to using a tube rectifier.

At this point, Jess Oliver switched to a 4-pin XLR instead of the octal speaker connector jack on the cabinet, which offered protective standby switching much like its predecessor and functioned as a fail-safe standby mechanism to prevent players from operating the amp with no load when the speaker was disconnected from the amplifier’s head. Functionally, the two features operate similarly, but you can always spot a B-15NB by the presence of an XLR speaker jack, blue-checkered vinyl, and a lack of an impedance switch on the back panel.

B-15NC

The B-15NC is most easily distinguished under the hood by its printed circuit board, double-baffled cabinet structure (the last of its breed), and a reintroduced tube rectifier. Toward the end of 1964, the B-15NB regained its rectifier tube and appropriate power transformer, and was re-dubbed the B-15NC. Instead of reverting to the 5U4GB rectifier used in the primitive B-15, Oliver chose to use a 5AR4 tube rectifier that supplied less voltage drop or sag. The 5AR4 also pulled less electricity from the filament current, making it a much friendlier option for the circuit’s power transformer. Ampeg also decided to revert the power supply filter capacitors back to the 20/20/40/40μF multi-section can capacitor from the B-15N.

Ampeg B-15NC

The B-15NC also was one of the first (if not the first) Ampeg amp to use a printed circuit board, drastically optimizing production times. Similarly, Ampeg shifted the wiring behind its tone stack to a wired, plastic-coated, integrated circuit. These circuits were highly popular in hi-fi and television hardware at the time, and it’s unclear whether or not Ampeg special-ordered these circuit packets or opted for a standard off-the-shelf unit.

This updated B-15 still maintained its 25W power rating, yet the new model did drop voltage down. The early B-15NCs usually used Jensen C15Ns, but by 1965, Ampeg switched to a 15-inch CTS speaker with a huge square ceramic magnet. The CTS speakers were far more rugged than the Jensens and also provided much sturdier bass response.

Ampeg B-15NC

During 1963, Ampeg also began offering JBL D-130F speakers as a built-to-order factory upgrade, denoted by the model number B-15NL. As you can expect, these amps are extremely rare.

Around this time, Ampeg also introduced the B-18 N, a behemoth amplifier endorsed by artists like Roger Waters. Showcasing an 18-inch speaker, the B-18 ran on entirely different circuitry. The power tubes were 7027As, pumping out 50W of tube power. The preamp tubes also were different from the B-18s contemporaries, employing two 6SL7s, coupled with a 7199 phase inverter. The earlier models used a solid-state rectifier, which offered bassists less voltage loss on bass notes, but later versions used a 5AR4 rectifier tube. This model was discontinued in 1968, but made it onto Pink Floyd’s The Wall in 1979. Roger Waters only used the B-18N on the Los Angeles sessions, and opted for a Fender Bassman 50 on the Britannia Row sessions for the record.

Sometime in early 1965, Jess Oliver also modified the B-15NC’s power supply filter capacitors. Since this change was so minor, Ampeg decided against changing the model name entirely.

B-15NF

By mid-1965, Ampeg decided to make the switch to a single-baffled porting structure and fixed-bias power tubes. The switch to fixed-bias tubes offered a lot of benefits for a bass amplifier. For one, they have a firmer, cleaner sound, but diminished sustain and dynamic responsiveness. Fixed-bias tubes also require biasing when changing power tubes, and they also need to be checked from time to time to ensure that the amp is responding properly. Finally, cathode or self-biased amps are less efficient. For example, a self-biased 6L6s Bassman-type amp would be about 25 watts, whereas the same circuit with fixed-bias 6L6s would be 40 watts. This updated B-15 now sported 30 watts instead of its 25 watt predecessors.

Ampeg B-15NF

Consequently, the B-15NF’s power section was altered to support fixed-bias operation and the necessary components to support that construction. The NF also featured a printed circuit board, and a single-baffle design that placed the baffle a quarter inch away from the front of the cabinet, slightly altering the speaker’s sound and response.

In 1967, Ampeg staged a beauty makeover, exchanging the company’s blue diamond vinyl with smooth black vinyl with chrome trim around the speaker grille opening. This would also be the last B-15 with both the light up Lucite logo and the chrome-plated chassis.

Check out this picture of Howlin’ Wolf and his bass player, better known as Andrew “Blueblood” McMahon, at the 1966 Newport Folk Festival. You can plainly see the B-15NF featured right next to the Wolf himself.

Howlin’ Wolf

B15-NC, B-15ND

Around the exact same time, Ampeg introduced the B-15NC and the B-15ND as a response to the increasing volume demands of performance artists.

The NC and ND borrowed a few aspects from the behemoth B-18, using a 7027A power amp section to churn out 50W of power. The NC featured a column-style cabinet (in fact, the “C” stands for column) that housed two 15-inch CTS speakers.

Ampeg B15-NC
Ampeg B-15ND

The ND offered one 15-inch Portaflex cabinet and an extension speaker. Occasionally, Ampeg would use Altec speakers in these two units, but for the most part, these speakers were CTS.

1968

The last group of changes that Ampeg made to the tube-powered B-15 were made in 1968. From the jump, you can plainly see that the B15-NF’s aesthetic is different. There’s no light-up Lucite logo. There’s no blue-checkered vinyl (though that change started in 1967). Ampeg also traded chrome for aluminum, and the understated script nameplate for the new and current “A” logo. Behind the 1968-1977 era B-15N’s apparent visual modifications were a few extra technical bells and whistles worth mentioning.

First off, Ampeg added a second input jack for its second channel. Ampeg also introduced Ultra High and Ultra Low switches on each channel. These switches function like Pultec style EQs, meaning that they emphasize certain frequencies by reducing the directly adjacent frequencies. The Ultra Low switch is centered at 160Hz, and the Ultra High switch is centered at 8kHz.

This B-15N design was altered again in 1973, when Ampeg decided to use Eminence speakers instead of the CTS (though at that point, you could get an Altec 421A as a factory add-on). 1973 also marked two other changes to the B-15: one, Ampeg moved the standby switch from the front control plate to the back of the chassis, and two, Ampeg shifted to a Thiele ported cabinet that featured rectangular baffles along the bottom of the baffle board separated from the speaker by a shelf.

The final modification to the B-15 came in 1975, when Ampeg switched to a grounded power cord, rendering the polarity switch obsolete. The B-15N survived until around 1980, when Ampeg decided to concentrate its efforts on the highly popular SVT and V-4B until the B-15 was resurrected as the B-15N in 1986, the B-15T (a solid-state, 100W variant) in 1988, and the B-15R (all-tube, 100W variant) in 1997.

1971-77

1971 saw the production of a two-channel, larger B-15 — this one called the B-15S. These units utilized a solid-state rectifier and a pair of 7027A tubes, two 12AX7s, one 12DW7, and a 12AU7 phase inverter, borrowed from the circuitry in the V-4B. The B-15S’s tone control circuit also included a three-way response switch with Bass, Flat, and Guitar settings. It also featured a tilt-back rod, which it kept until its untimely demise in 1977.

Rick Danko, bass player for The Band, loved using his B-15S. Here’s a picture of him with guitarist Robbie Robertson at one of their last concerts. Danko would use the B-15S with The Band until their final concert, which is forever memorialized in the Scorsese documentary The Last Waltz. Rumor has it that Queens of the Stone Age guitarist Josh Homme also uses a B-15S in the studio, but you didn’t hear that from me.

Ampeg B-15S
The Band

You can see that a B-15 is not always the B-15 you’re thinking of. No matter where your tastes lie, chances are that your favorite band used one of these bad boys. A lot of players prefer the ‘70s models because of their cabinet construction and solid low-mid response. I swear by my 1966 B-15NF because of its slightly higher wattage, fixed-bias construction, and firm, crispy tone. Regardless of which model you can get your hands on, these amplifiers are highly coveted by all sorts of players, and for good reason.


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